White noise generators generate sound at frequencies across the spectrum and are used as sleep aids or to protect privacy by masking other sound, such as conversations. Some so-called “white noise generators” actually generate “red” or “pink noise,” which sounds less harsh. In red noise, the power decreases as the frequency increases, so that more of the noise is generated at lower frequencies. Many white noise generators use electronic circuitry to generate a desired noise spectrum, which is output through a speaker. Although many people prefer background noise when sleeping, studying, working, etc., electronically generated noise often sounds artificial. Additionally, many people can detect patterns in the generated noise which make it less effective as pure background sound.
As a result, many people employ mechanical devices to produce background noise. Bedroom fans, for example, have long been used to generate noise to help people sleep. Fans are especially desirable because they provide not only noise, but also airflow, allowing more effective body temperature regulation by convection and more effective evaporation of perspiration. As the speed of the fan is increased to move more air, the noise level is increased. The sound of a fan is determined by the structure of the fan and its speed. The sound is therefore fixed by the setting of the fan speed. This dependence of the noise on the amount of air movement is unfortunate, however, because it may be desirable to change the sound without changing the airflow, or to change the airflow without changing the sound.